Project for Security Programming class

This is a group project. Each group must create a solution to the following problem. You are open to use your creativity in designing the application.

"Company XYZ needs a system where users can chat securely. The messages can be seen only by registered user. User can login to the system and send messages where the messages can be seen by other registered user. There is an admin user who could register, de-register new users. Below are some of the technical details:

1. Login is done via a username, password combo.
2. Each posted message must have a username displayed to signify the user that posted the message.
3. Messages are never stored in users pc.
4. All user can only see messages for the past 7 days. Messages older than that will automatically be deleted."